The present invention relates to a gas sensing element for detecting the presence of a predetermined gas in the ambient atmosphere using a transition metal oxide whose resistance is dependent on the presence of that gas and its concentration.
Several types of gas sensing elements have conventionally been used to detect the presence of a predetermined gas in the ambient atmosphere or measure its concentration. Generally, these elements use as a functional (active) layer TiO.sub.2, ZnO, SnO.sub.3 or other transition metal oxides that undergo a change in their electrical resistance upon contact with the certain gas. In order to meet the requirements for a simpler construction and higher productivity, it has been proposed that such gas sensing elements be produced employing a hybrid technique, whereby an electrode layer is formed on a substrate made of a ceramic insulator and a functional layer containing one or more of the above-described transition metal oxides as the main component is formed on the electrode layer.
For achieving a higher gas sensitivity, the porosity of the functional layer in these gas sensing elements is desirably maintained in the range of 25 to 50%, and in order to attain this range of porosity, the functional layer is usually sintered at temperatures in a range of about 900 to 1200.degree. C.
When a gas sensing element produced by the method described above is used to detect a predetermined gas in a hot exhaust flow from an internal combustion engine or other burner, the sensing element itself is exposed to the hot gas, which inevitably causes the resintering of the functional layer. When this occurs, the porosity of the functional layer changes to such an extent that the performance of the sensing element is deteriorated.
Proposals have been made to prevent the resintering of the functional layer by the addition of Na.sub.2 O, Cr.sub.2 O.sub.3, A1.sub.2 O.sub.3 or ZrO.sub.2 (see, for example, Japanese Unexamined Published Pat. application No. 57143/1980). However, these anti-sintering agents react with the transition metal oxide and cause an undesired change in the sensor's performance, such as an increased bulk resistivity.